Diversity combining method, and receiver

ABSTRACT

The invention relates to a diversity combining method and to a receiver. In diversity combining, outputs ( 33 ) of a matched filter ( 25 ) of each branch are weighted with a quality estimate ( 32 ) which is generated in quality means ( 28   b ) in such a matter that the quality estimate ( 30 ) is proportional to the inverse of the interference strength ( 31 ) of the signal. The strength ( 31 ) of signal interference is generated in interference means ( 28   c ) e.g. in variance-like fashion from the differences between a reference signal ( 30 ) generated in reference signal means ( 28   a ) as the convolution of the estimated channel impulse response and the predetermined sequence and a signal received from the channel.

The invention relates to a diversity combining method in a digital radiosystem receiver, in which receiver matched filtering and maximumlikelihood detection are used and an estimated channel impulse responseand autocorrelation taps of the impulse response are generated, and inwhich radio system substantially all signal processing occurs as symbolsand a desired signal comprises a predetermined sequence.

The invention also relates to a receiver in a digital radio system, thereceiver comprising a matched filter, diversity branches and a maximumlikelihood detector, the receiver being arranged to generate anestimated channel impulse response and autocorrelation taps of theimpulse response, and in which radio system a desired signal comprises apredetermined sequence and in which radio system signal processing isarranged to occur as symbols.

In a radio system the quality of the connection between a base stationand a subscriber terminal varies continuously. This variation is due tointerfering factors on the radio path and to attenuation of the radiowaves as a function of distance and time in a fading channel. Connectionquality can be measured for example by observing the received power.Variance in connection quality can partly be compensated by powerregulation.

However, in a digital radio system a more precise method than powermeasuring is needed for estimating connection quality. Then the knownquality parameters are for example the bit error rate (BER) and thesignal-to-noise ratio.

It is previously known to utilize decisions of the ML (MaximumLikelihood) type detection for estimating the signal-to-noise ratio of areceived signal. Thus a Viterbi detector usually functions as the MLdetector and a base station or subscriber terminal can be the receiver.In known solutions the Viterbi detection is performed on the receivedburst in full before determining the signal-to-noise ratio. However, asa Viterbi algorithm is often a too demanding measure for a digitalsignal processing program to perform during the processing time allowedby the receiver, separate Viterbi hardware has to be used. This has beendescribed in greater detail in J. Hagenauer, P. Hoeher: A ViterbiAlgorithm with Soft-decision Outputs and its Applications, IEEE GLOBECOM1989, Dallas, Tex., November 1989, which is incorporated herein byreference.

It is known that a signal quality estimate, often the signal-to-noiseratio, is needed when using different diversity receivers. In diversityreception the most common diversity receivers combine the signals beforeor after detection and comprise e.g. selective combining, maximal-ratiocombining and equal-gain combining. The diversity signals are usuallydetected using a Viterbi detector, the signals being combined afterdetection. However, it is preferable to combine the signals beforedetection, thus achieving a greater amplification of the signal.Diversity receivers have been described in greater detail for example inthe book William C. Y. Lee: Mobile Communications Engineering, chapter10, Combining technology, pages 291-336, McGraw-Hill, USA, 1982, whichis incorporated herein by reference.

An object of the present invention is to implement a method forestimating the interference strength directly from a received signalwithout the help of ML detection and simultaneously enabling thecombination of diversity signals before detection when using diversityreceivers.

This is achieved with the method set forth in the preamble characterizedin that a reference signal is generated from the estimated channelimpulse response and the predetermined sequence by convolution; aninterference strength connected to the desired signal is generated usingthe differences of the reference signal and the predetermined sequencereceived from the channel; a strength value of the desired signal isgenerated, whereby a quality estimate is generated by dividing thestrength value of the desired signal by the interference strength of thedesired signal; and diversity combining is performed in such a mannerthat the symbols of the different branches corresponding with each otherin the time domain are combined, and the outputs of the matched filtersof each branch and the autocorrelation taps of the impulse response areweighted with the quality estimate of each branch.

The receiver of the invention is characterized in that the receivercomprises reference signal means for generating a reference signal fromthe estimated channel impulse response and the predetermined sequence byconvolution; interference means for generating the interference strengthassociated with the desired signal using the differences of thereference signal and the predetermined sequence received from thechannel; the receiver is arranged to generate a strength value of thedesired signal and quality means are arranged to generate a qualityestimate by dividing the strength value of the desired signal by theinterference strength of the desired signal; and combining means of thediversity branches combine the symbols of the different branchescorresponding to each other in the time domain, and that the receiver isarranged to weight the matched filter outputs of each branch and theautocorrelation taps of the impulse response with the quality estimateof each branch.

Great advantages are achieved with the invention. With the method of theinvention the interference strength can be estimated directly from thereceived signal without performing a Viterbi detection. By avoiding theuse of the Viterbi algorithm usually applied to the ML method memory andtime used for calculating are saved. The generated interference strengthcan be utilized for estimating the status of the channel, as help inmethods of estimating bad frames and for scaling the ML metric.Furthermore, the interference strength can be utilized for diversitycombining and it is particularly useful when multipath signals arecombined before detection.

In the following, the invention will be described in greater detail withreference to examples in the accompanying drawings, in which

FIG. 1 shows a radio system,

FIG. 2 shows a normal burst of the GSM system,

FIG. 3 shows a block diagram of the receiver,

FIG. 4 shows a block diagram of the receiver and

FIG. 5 shows a receiver using a diversity combining technique.

The method and the receiver of the invention can be applied to the GSMradio system (Global System for Mobile communication) withoutrestricting it thereto. In FIG. 1 the radio system comprises basestation 1, and a number of generally moving subscriber terminals 2-4having bi-directional connections 6-8 with the base station. Basestation 1 transmits the connections of terminals 2-4 to base stationcontroller 5 which transmits them further to other parts of the systemand if necessary to a fixed network. Base station controller 5 controlsthe function of one or several base stations 1. In the GSM system bothbase station 1 and terminals 2-4 constantly measure connection quality.

Let us now examine in more detail the solution of the invention in theGSM system. A normal burst of the GSM system is shown in FIG. 2, theburst comprising 148 symbols in all. The symbols comprise bits and bitcombinations. The symbols of the burst are arranged in sequencescomprising 3 start symbols (TS) 10, 58 data symbols (Data) 11, 26training symbols (TRS) 12, 58 data symbols (Data) 13 and 3 end symbols(TS) 14. In the solution of the invention the symbol sequence of thereference signal is calculated as the function of training sequence 12and of the estimated channel impulse response, preferably being theconvolution of said sequences. The generation of convolution functionh(t) can be shown in the following way between functions f(t) and g(t)in its general form: $\begin{matrix}{{h(t)} = {{\left( {f*g} \right)(t)} = {\int_{0}^{t}{{f(\tau)}{g\left( {t - \tau} \right)}\quad {{\tau}.}}}}} & (1)\end{matrix}$

In the following, one method of the invention is described when applyingit to the GSM system in particular. The calculation of the channel'smomentary quality estimate QE comprises two essential steps: firstly thegeneration of reference signal YR from estimated channel impulseresponse H and training sequence TRS (training symbols 12 in FIG. 2)preferably as a convolution and secondly the generation of theinterference strength for example as interference energy VAR fromreference signal YR and training sequence Y received from the channel ina variance-like fashion. By calculating the convolution such anadvantage is achieved that reference signal YR is generated in the sameway as the actual signal on the channel and by comparing this resuitwith the desired signal received from the channel the interferencestrength can be estimated. Variance VAR is generally calculated for thediscreet distribution in the following way: $\begin{matrix}{{\delta^{2} = {{VAR} = {\sum\limits_{j}\quad {\left( {x_{j} - \mu} \right)^{2}{f\left( x_{j} \right)}}}}},} & (2)\end{matrix}$

where μ is an expected value. The interference strength can also bedetermined for example in a standard deviation-like fashion. Standarddeviation δ is according to its definition the positive square root ofvariance δ². Furthermore, quadratic difference (x_(j)−μ)² can in themethod of the invention be replaced by any exponent |x_(j)−μ|^(z) of theabsolute value of the difference where z is any real number. Wheninterference strength VAR is calculated from reference signal YR andfrom the signal received from the channel in a variance type fashion,such an advantage is achieved that the result obtained is directly theeffective value of the interference.

As training sequence TRS is predetermined it is possible to determinemomentary estimated channel impulse response H. Usually estimatedimpulse response H has 5 symbols i.e. N=5 is valid for the number ofsymbols N. In the first step of the method of the invention referencesignal YR, which is the expected value of received training sequence TRSwith said estimated impulse response H, is calculated e.g. according toformula (3) as the convolution of estimated channel impulse response Hand training sequence TRS. $\begin{matrix}{{{YR}(j)} = {\sum\limits_{i = 0}^{N - 1}\quad {{{HU}(i)} \cdot \left( {1 - {2 \cdot {{TRS}\left( {j - i} \right)}}} \right)}}} & (3)\end{matrix}$

where N is the number of symbols in estimated impulse response H and j≧Nis valid for symbol index j which shows the symbol to be calculated.Entire reference signal YR is obtained by going through the symbols jbetween N and 26 or the number of symbols in a predetermined sequence.Using obtained reference signal YR and received signal Y comprising thetraining sequence, their variance-type interference strength VAR iscalculated for example by using formula (4). $\begin{matrix}{{VAR} = \frac{{\sum\limits_{i = N}^{26}\quad {{Re}\left( {{Y\left( {i + {offset}} \right)} - {{YR}(i)}} \right)}^{2}} + {{lm}\left( {{Y\left( {i + {offset}} \right)} - {{YR}(i)}} \right)}^{2}}{K}} & (4)\end{matrix}$

The maximum number of symbols taken into account in formula (4) is thenumber of symbols of predetermined sequence 12 less the number ofsymbols in the estimated channel impulse response. Then the number ofsymbols considered in the calculation can be freely chosen. Interferencestrength VAR is thus calculated as variance, but number K in itsnominator is not significant, as the nominator simply has to be formedand it only functions as the scalar of the interference strength. Thisis easy to observe and to correct in any step of generating the qualityestimate. In formula (4) the value of variance-type result VAR is thesame as the interference energy per sample if the number of symbols usedin summing is set as the value of divisor K, or the energy per entiresequence if the value of divisor K is one. In formula (4) I/Q modulationmarkings are used the symbols being shown in their complex mode. Anoffset is also observed in formula (4) i.e. it is preferable to transferthe symbols of the received signal in such a manner that the symbol ofthe received signal corresponds with the symbol of the reference signal.

The strength value E of the received desired signal, which value can bethe amplitudinal strength of the symbols to be considered in the summingor the effective value or another corresponding exponent of the symbol'samplitude, can be calculated either using the estimated channel impulseresponse H, using reference signal YR or using the desired signalreceived from the channel. The advantage of calculating the effectivevalue of the taps of estimated channel impulse response H is that energyE of the signal is obtained per symbol. When energy E is calculatedusing the complex symbols of the I/Q modulation of the reference signale.g. using formula (5) $\begin{matrix}{E = {{\sum\limits_{i = N}^{26}\quad {{Re}\left( {{YR}(i)} \right)}^{2}} + {{lm}\left( {{YR}(i)} \right)}^{2}}} & (5)\end{matrix}$

the entire energy of the reference signal is obtained directly. Theenergy of the signal received from the channel can be calculatedsimilarly. If the normalized average energy of the signal correspondingto energy E, is formed by preprocessing means 24 according to prior art,it does not have to be separately calculated. The strength value of thedesired signal is directly calculated from the desired signal as informula (5), but the symbols of reference signal YR are replaced withthe symbols of desired signal Y.

Momentary channel quality estimate QE is preferably obtained by formingthe inverse of the interference strength as shown in formula (6)$\begin{matrix}{{QE} = {\frac{1}{VAR}.}} & (6)\end{matrix}$

Quality estimate QE can also be formed as shown in formula (7) by way ofprinciple by dividing signal energy per symbol E by noise energy persymbol VAR when the number of symbols is used as the value of divisor Kin formula (4). $\begin{matrix}{{QE} = \frac{E}{VAR}} & (7)\end{matrix}$

Another preferable way of calculating quality estimate QE according toformula (7) is to divide entire signal energy E by entire noise energyVAR, when the value of divisor K in formula (4) is one, and to avoidunnecessary dividing as formulas (4) and (5) comprise in this case asubstantially equal amount of elements to be summed. When the receivercomprises several diversity branches, the signal components of thedifferent diversity branches and the autocorrelation taps of theestimated impulse response are weighted with quality estimate QE of eachbranch. The weighting preferably occurs by multiplying the signalcomponents in the matched filter by the autocorrelation taps of theestimated impulse response as they are being generated.

Quality estimate QE is preferably calculated separately for each burstas the connection quality differs greatly even during a short time.

Let us now examine in greater detail the receiver of the cellular radiosystem of the invention, the block diagram of which is shown withessential parts in FIG. 3. Both a base station and a subscriber terminalcan function as the receiver of the invention. The receiver comprisesantenna 21 for conveying a received desired signal to radio frequencyparts 22 in which the signal is converted into an intermediatefrequency. From the radio frequency parts the signal is conveyed intoconversion means 23 in which the signal is converted from analog intodigital form. The digital signal propagates to preprocessing means 24where the signal can e.g. be filtered, a DC offset can be removed fromit, an automatic amplification of the digital signal can be controlledand the signal can be demodulated. Filter 25 matched to the channelrestores the signal distorted on the channel to the original data flowwith a low symbol error probability. The channel impulse responseestimate and its effective value are generated by means 26.Autocorrelation taps 34 of the estimated channel impulse response aregenerated from the impulse response information using means 27.

In the digital radio system the channel impulse response is describedwith a number comprising N symbols. The channel impulse response usuallycomprises five symbols, i.e. N obtains the value 5. Quality estimate 32(QE) is calculated with the method of the invention using means 28comprising means 28 a, 28 b and 28 c. Reference signal means 28 a areused for generating reference signal 30 (YR) from the estimated channelimpulse response and the predetermined sequence comprised in the signal.Interference means 28 b generate interference strength 31 from thedifference between reference signal YR and the received predeterminedsequence. Quality means 28 c generate quality estimate QE of theconnection of the invention in such a manner that the quality estimateis reversed proportional to the interference connected to the desiredsignal. The receiver is also arranged to correct the offset, or thesignal, of reference signal 30 and the predetermined sequence, i.e. thetransfer in time of the symbols in relation to one another caused bypropagation delay. Finally ML detection means 29 of the receiver,preferably a Viterbi detector, receive output 33 of matched filter 25i.e. the different sequences of the received burst shown in FIG. 2 andautocorrelation taps 34 of the channel impulse response from means 27.In this receiver solution both autocorrelation taps 34 of the impulseresponse in means 27 and the desired signal in matched filter 25 areweighted by quality estimate QE. The detected symbols are the output ofML detection means 29.

Let us now examine in greater detail a second radio system receiver ofthe invention as an alternative to the first, the block diagram of whichis shown with essential parts in FIG. 4. The receiver is, to a verylarge extent, similar to the receiver in FIG. 3. In this receiversolution quality estimate 32 (QE) is transferred from quality means 28 cto means 26, which generates an impulse response weighted by qualityestimate QE, whereby quality estimate QE affects output 33 of matchedfilter 25 and weights the desired signal being processed. The detectedsymbols are the output of ML detection means 29.

The solutions shown in FIGS. 3 and 4 can preferably be utilized inmultipath reception, FIG. 5 showing such an arrangement, when thereceiver uses diversity combining. The receiver in FIG. 5 comprises twodiversity branches 50, 51, both of which comprising antenna 41 and 42,means 43 and 44, which in turn comprise e.g. radio frequency parts 22,conversion means 23, preprocessing means 24, matched filters 25, channelimpulse response estimation means 26, calculation means 28 of thequality estimate, as do the receivers in FIGS. 3 and 4. Although FIG. 5shows only two diversity branches, or channels, 50, 51, similardiversity combining can also be applied to several channels. Estimatedimpulse response autocorrelation taps 34 of the different channels aregenerated by means 27, which represent the same function as means 27 ofFIGS. 3 and 4. The signal components arriving from different channels,and that are outputs 33 of matched filter 25, are combined by means 45where the combining is performed e.g. by summing or averaging and whenso desired by multiplying the signals by a suitable constant. In thesolution of the invention the signal component of each diversity branchis weighted by quality estimate QE of the particular branch 50, 51. In asteep weighting only the best signal component or the best signalcomponents are selected for detector 29 on the basis of quality estimateQE. After combining the signal is conveyed to ML detection means 29.Also outputs 34 of generating means 27 of the impulse responseautocorrelation taps are combined by means 46 e.g. by summing oraveraging and when so desired by multiplying the signals by a suitableconstant. In combining diversity branches 50, 51 and autocorrelationtaps 34 it is preferable to combine only the symbols or the bits thatcorrespond with one another in time. The output of means 46 is alsoconveyed to ML detection means 29. Such a solution is particularlyuseful because a greater amplification of the signal is achieved whenthe signal components are combined before detection.

The solutions of the invention can be implemented particularly regardingdigital signal processing with e.g. ASIC or VLSI circuits. The functionsto be performed are preferably implemented as programs based onmicroprocessor technology.

Even though the invention has been described above with reference to theexample of the accompanying drawings, it is obvious that the inventionis not restricted to it but can be modified in various ways within thescope of the inventive idea disclosed in the attached claims.

What is claimed is:
 1. A diversity combining method in a digital radiosystem receiver (1-4), in which receiver matched filtering and maximumlikelihood detection are used and an estimated channel impulse responseand autocorrelation taps of the impulse response are generated, and inwhich radio system substantially all signal processing occurs as symbolsand a desired signal comprises a predetermined sequence (12),characterized in that a reference signal (30) is generated from theestimated channel impulse response and the predetermined sequence (12)by convolution; an interference strength (31) connected to the desiredsignal is generated using the differences of the reference signal (30)and the predetermined sequence (12) received from the channel; astrength value (35) of the desired signal is generated, whereby aquality estimate (32) is generated by dividing the strength value (35)of the desired signal by the interference strength (31) of the desiredsignal; and diversity combining is performed in such a manner that thesymbols of the different branches (50, 51) corresponding with each otherin the time domain are combined, and the outputs (33) of the matchedfilters (25) of each branch (50, 51) and the autocorrelation taps of theimpulse response are weighted with the quality estimate (32) of eachbranch (50, 51).
 2. A method as claimed in claim 1, characterized inthat the diversity combining occurs before the maximum likelihooddetection.
 3. A method as claimed in claim 1, characterized in that theinterference strength (31) associated with the desired signal ispreferably generated in a variance-like or a standard deviation-likefashion from the differences between the reference signal (31) and thepredetermined sequence (12) of the desired signal and the strength value(35) of the desired signal is generated as the sum, the quadratic sum oranother corresponding power sum of the symbols strengths of thereference signal (30), the desired signal or the taps (34) of theestimated channel impulse response.
 4. A method as claimed in claim 1,characterized in that when the transmission occurs in bursts the qualityestimate (32) is calculated separately for each received burst.
 5. Amethod as claimed in claim 1, characterized in that when the radiosystem is the GSM system, the predetermined sequence (12) is thetraining sequence of a normal burst of the GSM system.
 6. A receiver(10-13) in a digital radio system, the receiver comprising a matchedfilter (25), diversity branches (50, 51) and a maximum likelihooddetector (29), the receiver being arranged to generate an estimatedchannel impulse response and autocorrelation taps (34) of the impulseresponse, and in which radio system a desired signal comprises apredetermined sequence (12) and in which radio system signal processingis arranged to occur as symbols, characterized in that the receiver(10-13) comprises reference signal means (28 a) for generating areference signal (30) from the estimated channel impulse response andthe predetermined sequence (12) by convolution; interference means (28c) for generating the interference strength (31) associated with thedesired signal using the differences of the reference signal (30) andthe predetermined sequence (12) received from the channel; the receiveris arranged to generate a strength value (35) of the desired signal andquality means (28 b) are arranged to generate a quality estimate (32) bydividing the strength value (35) of the desired signal by theinterference strength (31) of the desired signal; and combining means(45, 46) of the diversity branches (50, 51) combine the symbols of thedifferent branches (50, 51) corresponding to each other in the timedomain, and that the receiver is arranged to weight the matched filter(25) outputs (33) of each branch (50, 51) and the autocorrelation tapsof the impulse response with the quality estimate of each branch (50,51).
 7. A receiver (10-13) as claimed in claim 6, characterized in thatthe combining means (45 and 46) of the diversity branches (50, 51) arelocated before the maximum likelihood detection means (29).
 8. Areceiver as claimed in claim 6, characterized in that the interferencemeans (28 c) are arranged to generate the interference strength (31)associated with the desired signal preferably in a variance-like orstandard deviation-like fashion from the differences between thereference signal (30) and the predetermined sequence (12) of the desiredsignal and the receiver is arranged to generate the strength value (35)of the desired signal using the sum, the quadratic sum or anothercorresponding power sum of the symbols strengths of the reference signal(30), the desired signal or the taps (34) of the estimated channelimpulse response.
 9. A receiver as claimed in claim 6, characterized inthat when the transmission occurs in bursts the receiver is arranged togenerate the quality estimate (32) separately for each received burst.10. A receiver as claimed in claim 6, characterized in that when theradio system is the GSM system the predetermined sequence (12) is thetraining sequence of a normal burst of the GSM system.